--- Manipulating iterators as sequences.
-- See @{07-functional.md.Sequences|The Guide}
--
-- Dependencies: `tagen.core`, `debug`
-- @module tagen.seq

local next,assert,type,pairs,tonumber,type,setmetatable,getmetatable,_G = next,assert,type,pairs,tonumber,type,setmetatable,getmetatable,_G
local strfind,strmatch,format = string.find,string.match,string.format
local mrandom = math.random
local remove,tsort,tappend = table.remove,table.sort,table.insert
local io = io
local tagen = require 'tagen.core'
local function_arg = tagen.function_arg
local _List = tagen.stdmt.List
local _Map = tagen.stdmt.Map
local assert_arg = tagen.assert_arg
local debug = require 'debug'

local seq = {}

-- given a number, return a function(y) which returns true if y > x
-- @param x a number
function seq.greater_than(x)
  return function(v)
  return tonumber(v) > x
  end
end

-- given a number, returns a function(y) which returns true if y < x
-- @param x a number
function seq.less_than(x)
  return function(v)
  return tonumber(v) < x
  end
end

-- given any value, return a function(y) which returns true if y == x
-- @param x a value
function seq.equal_to(x)
  if type(x) == "number" then
  return function(v)
    return tonumber(v) == x
  end
  else
  return function(v)
    return v == x
  end
  end
end

--- given a string, return a function(y) which matches y against the string.
-- @param s a string
function seq.matching(s)
  return function(v)
   return strfind(v,s)
  end
end

local nexti

--- sequence adaptor for a table.   Note that if any generic function is
-- passed a table, it will automatically use seq.list()
-- @param t a list-like table
-- @usage sum(list(t)) is the sum of all elements of t
-- @usage for x in list(t) do...end
function seq.list(t)
  assert_arg(1,t,'table')
  if not nexti then
  nexti = ipairs{}
  end
  local key,value = 0
  return function()
  key,value = nexti(t,key)
  return value
  end
end

--- return the keys of the table.
-- @param t an arbitrary table
-- @return iterator over keys
function seq.keys(t)
  assert_arg(1,t,'table')
  local key,value
  return function()
  key,value = next(t,key)
  return key
  end
end

local list = seq.list
local function default_iter(iter)
  if type(iter) == 'table' then return list(iter)
  else return iter end
end

seq.iter = default_iter

--- create an iterator over a numerical range. Like the standard Python function xrange.
-- @param start a number
-- @param finish a number greater than start
function seq.range(start,finish)
  local i = start - 1
  return function()
    i = i + 1
    if i > finish then return nil
    else return i end
  end
end

-- count the number of elements in the sequence which satisfy the predicate
-- @param iter a sequence
-- @param condn a predicate function (must return either true or false)
-- @param optional argument to be passed to predicate as second argument.
-- @return count
function seq.count(iter,condn,arg)
  local i = 0
  seq.foreach(iter,function(val)
    if condn(val,arg) then i = i + 1 end
  end)
  return i
end

--- return the minimum and the maximum value of the sequence.
-- @param iter a sequence
-- @return minimum value
-- @return maximum value
function seq.minmax(iter)
  local vmin,vmax = 1e70,-1e70
  for v in default_iter(iter) do
  v = tonumber(v)
  if v < vmin then vmin = v end
  if v > vmax then vmax = v end
  end
  return vmin,vmax
end

--- return the sum and element count of the sequence.
-- @param iter a sequence
-- @param fn an optional function to apply to the values
function seq.sum(iter,fn)
  local s = 0
  local i = 0
  for v in default_iter(iter) do
  if fn then v = fn(v) end
  s = s + v
  i = i + 1
  end
  return s,i
end

--- create a table from the sequence. (This will make the result a List.)
-- @param iter a sequence
-- @return a List
-- @usage copy(list(ls)) is equal to ls
-- @usage copy(list {1,2,3}) == List{1,2,3}
function seq.copy(iter)
  local res,k = {},1
  for v in default_iter(iter) do
    res[k] = v
    k = k + 1
  end
  setmetatable(res,_List)
  return res
end

--- create a table of pairs from the double-valued sequence.
-- @param iter a double-valued sequence
-- @param i1 used to capture extra iterator values
-- @param i2 as with pairs & ipairs
-- @usage copy2(ipairs{10,20,30}) == {{1,10},{2,20},{3,30}}
-- @return a list-like table
function seq.copy2 (iter,i1,i2)
  local res,k = {},1
  for v1,v2 in iter,i1,i2 do
    res[k] = {v1,v2}
    k = k + 1
  end
  return res
end

--- create a table of 'tuples' from a multi-valued sequence.
-- A generalization of copy2 above
-- @param iter a multiple-valued sequence
-- @return a list-like table
function seq.copy_tuples (iter)
  iter = default_iter(iter)
  local res = {}
  local row = {iter()}
  while #row > 0 do
    tappend(res,row)
    row = {iter()}
  end
  return res
end

--- return an iterator of random numbers.
-- @param n the length of the sequence
-- @param l same as the first optional argument to math.random
-- @param u same as the second optional argument to math.random
-- @return a sequnce
function seq.random(n,l,u)
  local rand
  assert(type(n) == 'number')
  if u then
   rand = function() return mrandom(l,u) end
  elseif l then
   rand = function() return mrandom(l) end
  else
   rand = mrandom
  end

  return function()
   if n == 0 then return nil
   else
     n = n - 1
     return rand()
   end
  end
end

--- return an iterator to the sorted elements of a sequence.
-- @param iter a sequence
-- @param comp an optional comparison function (comp(x,y) is true if x < y)
function seq.sort(iter,comp)
  local t = seq.copy(iter)
  tsort(t,comp)
  return list(t)
end

--- return an iterator which returns elements of two sequences.
-- @param iter1 a sequence
-- @param iter2 a sequence
-- @usage for x,y in seq.zip(ls1,ls2) do....end
function seq.zip(iter1,iter2)
  iter1 = default_iter(iter1)
  iter2 = default_iter(iter2)
  return function()
    return iter1(),iter2()
  end
end

--- Makes a table where the key/values are the values and value counts of the sequence.
-- This version works with 'hashable' values like strings and numbers.
-- `tagen.tablex.count_map` is more general.
-- @param iter a sequence
-- @return a map-like table
-- @return a table
-- @see tagen.tablex.count_map
function seq.count_map(iter)
  local t = {}
  local v
  for s in default_iter(iter) do
    v = t[s]
    if v then t[s] = v + 1
    else t[s] = 1 end
  end
  return setmetatable(t,_Map)
end

-- given a sequence, return all the unique values in that sequence.
-- @param iter a sequence
-- @param returns_table true if we return a table, not a sequence
-- @return a sequence or a table; defaults to a sequence.
function seq.unique(iter,returns_table)
  local t = seq.count_map(iter)
  local res,k = {},1
  for key in pairs(t) do res[k] = key; k = k + 1 end
  table.sort(res)
  if returns_table then
    return res
  else
    return list(res)
  end
end

--- print out a sequence iter with a separator.
-- @param iter a sequence
-- @param sep the separator (default space)
-- @param nfields maximum number of values per line (default 7)
-- @param fmt optional format function for each value
function seq.printall(iter,sep,nfields,fmt)
  local write = io.write
  if not sep then sep = ' ' end
  if not nfields then
    if sep == '\n' then nfields = 1e30
    else nfields = 7 end
  end
  if fmt then
  local fstr = fmt
  fmt = function(v) return format(fstr,v) end
  end
  local k = 1
  for v in default_iter(iter) do
   if fmt then v = fmt(v) end
   if k < nfields then
     write(v,sep)
     k = k + 1
  else
     write(v,'\n')
     k = 1
  end
  end
  write '\n'
end

-- return an iterator running over every element of two sequences (concatenation).
-- @param iter1 a sequence
-- @param iter2 a sequence
function seq.splice(iter1,iter2)
  iter1 = default_iter(iter1)
  iter2 = default_iter(iter2)
  local iter = iter1
  return function()
  local ret = iter()
  if ret == nil then
    if iter == iter1 then
    iter = iter2
    return iter()
    else return nil end
   else
     return  ret
   end
 end
end

--- return a sequence where every element of a sequence has been transformed
-- by a function. If you don't supply an argument, then the function will
-- receive both values of a double-valued sequence, otherwise behaves rather like
-- tablex.map.
-- @param fn a function to apply to elements; may take two arguments
-- @param iter a sequence of one or two values
-- @param arg optional argument to pass to function.
function seq.map(fn,iter,arg)
  fn = function_arg(1,fn)
  iter = default_iter(iter)
  return function()
    local v1,v2 = iter()
    if v1 == nil then return nil end
    if arg then return fn(v1,arg) or false
    else return fn(v1,v2) or false
    end
  end
end

--- filter a sequence using a predicate function.
-- @param iter a sequence of one or two values
-- @param pred a boolean function; may take two arguments
-- @param arg optional argument to pass to function.
function seq.filter (iter,pred,arg)
  pred = function_arg(2,pred)
  return function ()
    local v1,v2
    while true do
      v1,v2 = iter()
      if v1 == nil then return nil end
      if arg then
        if pred(v1,arg) then return v1,v2 end
      else
        if pred(v1,v2) then return v1,v2 end
      end
    end
  end
end

--- 'reduce' a sequence using a binary function.
-- @param fun a function of two arguments
-- @param iter a sequence
-- @param oldval optional initial value
-- @usage seq.reduce(operator.add,seq.list{1,2,3,4}) == 10
-- @usage seq.reduce('-',{1,2,3,4,5}) == -13
function seq.reduce (fun,iter,oldval)
   fun = function_arg(1,fun)
   iter = default_iter(iter)
   if not oldval then
     oldval = iter()
   end
   local val = oldval
   for v in iter do
     val = fun(val,v)
   end
   return val
end

--- take the first n values from the sequence.
-- @param iter a sequence of one or two values
-- @param n number of items to take
-- @return a sequence of at most n items
function seq.take (iter,n)
  local i = 1
  iter = default_iter(iter)
  return function()
    if i > n then return end
    local val1,val2 = iter()
    if not val1 then return end
    i = i + 1
    return val1,val2
  end
end

--- skip the first n values of a sequence
-- @param iter a sequence of one or more values
-- @param n number of items to skip
function seq.skip (iter,n)
  n = n or 1
  for i = 1,n do iter() end
  return iter
end

--- a sequence with a sequence count and the original value.
-- enum(copy(ls)) is a roundabout way of saying ipairs(ls).
-- @param iter a single or double valued sequence
-- @return sequence of (i,v), i = 1..n and v is from iter.
function seq.enum (iter)
  local i = 0
  iter = default_iter(iter)
  return function  ()
    local val1,val2 = iter()
    if not val1 then return end
    i = i + 1
    return i,val1,val2
  end
end

--- map using a named method over a sequence.
-- @param iter a sequence
-- @param name the method name
-- @param arg1 optional first extra argument
-- @param arg2 optional second extra argument
function seq.mapmethod (iter,name,arg1,arg2)
  iter = default_iter(iter)
  return function()
    local val = iter()
    if not val then return end
    local fn = val[name]
    if not fn then error(type(val).." does not have method "..name) end
    return fn(val,arg1,arg2)
  end
end

--- a sequence of (last,current) values from another sequence.
--  This will return S(i-1),S(i) if given S(i)
-- @param iter a sequence
function seq.last (iter)
  iter = default_iter(iter)
  local l = iter()
  if l == nil then return nil end
  return function ()
    local val,ll
    val = iter()
    if val == nil then return nil end
    ll = l
    l = val
    return val,ll
  end
end

--- call the function on each element of the sequence.
-- @param iter a sequence with up to 3 values
-- @param fn a function
function seq.foreach(iter,fn)
  fn = function_arg(2,fn)
  for i1,i2,i3 in default_iter(iter) do fn(i1,i2,i3) end
end

---------------------- Sequence Adapters ---------------------

local SMT
local callable = tagen.is_callable

local function SW (iter,...)
  if callable(iter) then
    return setmetatable({iter=iter},SMT)
  else
    return iter,...
  end
end


-- can't directly look these up in seq because of the wrong argument order...
local map,reduce,mapmethod = seq.map, seq.reduce, seq.mapmethod
local overrides = {
  map = function(self,fun,arg)
    return map(fun,self,arg)
  end,
  reduce = function(self,fun)
    return reduce(fun,self)
  end
}

SMT = {
  __index = function (tbl,key)
    local fn = overrides[key] or seq[key]
    if fn then
      return function(sw,...) return SW(fn(sw.iter,...)) end
    else
      return function(sw,...) return SW(mapmethod(sw.iter,key,...)) end
    end
  end,
  __call = function (sw)
    return sw.iter()
  end,
}

setmetatable(seq,{
  __call = function(tbl,iter)
    if not callable(iter) then
      if type(iter) == 'table' then iter = seq.list(iter)
      else return iter
      end
    end
    return setmetatable({iter=iter},SMT)
  end
})

--- create a wrapped iterator over all lines in the file.
-- @param f either a filename, file-like object, or 'STDIN' (for standard input)
-- @param ... for Lua 5.2 only, optional format specifiers, as in `io.read`.
-- @return a sequence wrapper
function seq.lines (f,...)
  local n = select('#',...)
  local iter,obj
  if f == 'STDIN' then
    f = io.stdin
  elseif type(f) == 'string' then
    iter,obj = io.lines(f,...)
  elseif not f.read then
    error("Pass either a string or a file-like object",2)
  end
  if not iter then
    iter,obj = f:lines(...)
  end
  if obj then -- LuaJIT version returns a function operating on a file
    local lines,file = iter,obj
    iter = function() return lines(file) end
  end
  return SW(iter)
end

function seq.import ()
  _G.debug.setmetatable(function() end,{
    __index = function(tbl,key)
      local s = overrides[key] or seq[key]
      if s then return s
      else
        return function(s,...) return seq.mapmethod(s,key,...) end
      end
    end
  })
end

return seq
